1. Field of the Invention
The present invention relates to digital communication systems and, specifically, to wireless digital communication systems designed for improved performance such as power efficiency and bit error rate.
2. Background Overview
Wireless systems include satellite, cellular, fixed access, wireless LANs (local area networks) and personal AN (area networks). The trend in wireless systems involves integration of the various networks, increased data rates, and proliferation of services such as Internet, data and image transmission or downloads, and voice over IP (Internet Protocol). Thus, to accommodate this trend performance attributes accounted for in wireless communications systems include sensitivity, selectivity, dynamic range, data rate capacity, power efficiency, and bit error rates. In addition, manufacturing and marketing attributes such as low cost, high reliability, and flexibility are becoming increasingly important.
Digital wireless communication systems include a transmitter, a receiver (including a software-defined radio) or both (a combination that is referred to as “transceiver”). The block diagram of FIG. 1, shows typical transmitter and receiver components in a digital wireless communication system 10. In such a system there are a number of possible locations where distortion is a factor, including the baseband stage 11 & 12 (with digital data or low frequency such as <30 MHz), intermediate frequency stage 13 & 14 (IF, such as 2 GHz), and radio frequency stage 15, 16 & 17 (RF, such as 6 GHz) including mmW (millimeter wave frequencies, such as 38 GHz).
In digital communication systems, the enabling technology is a combination of software control, RF and IF circuitry, and digital circuitry, including signal-processing components. The design of digital communication systems presents a challenge of optimizing the RF circuit functionality to address one or more of the foregoing performance, manufacturing, and marketing attributes and the complexities they introduce.
For example, in the transmitter, the RF power amplifier is expected to meet peak and average power specifications and provide high power efficiency within the specified frequency range. However, distortion in amplifiers generates AM—AM and AM–PM non-linearities (AM stands for amplitude modulation, PM stands for phase modulation). Hence, communication systems using linear modulation techniques such as quadrature amplitude modulation (QAM) are restricted by the performance of the transmitter and receiver. One of the restrictions is the aforementioned nonlinear characteristic of the power amplifier that causes the AM—AM and AM–PM distortion.
There are a number of approaches for controlling linearity and distortion levels. The simplest approach involves using higher power devices in the power amplifier while operating at a high back-off ratio in the output power level. The drawbacks of this approach include increased DC power consumption, higher cost, and lower reliability.
A common approach for improving power amplifier linearity is to use RF signal feedback. In a higher frequency range, the tradeoff for improved linearity is reduced gain and, in turn, reduced power level. In transmitters, the reduction in output power level has an adverse effect on the allowable distance between the transmitters and corresponding receivers.
Baseband signal feedback—which is analogous to pre-distortion of power amplifier input—is used in transmitters to provide for some of the deficiencies of RF signal feedback. This approach involves baseband signal modulation of an RF carrier, and amplification of the modulated RF carrier signal by the non-linear RF power amplifier. A sample of the amplified, modulated carrier signal is demodulated and fed back to the input of a baseband amplifier where it is combined with the basedband input of that amplifier. The introduction of the demodulated sampled carrier signal at the input of the baseband amplifier creates a pre-distortion of the baseband signal to counteract the distortion from the RF power amplifier's non-linearity. The tradeoff in this case is a feedback loop delay that limits the possible bandwidth of transmitted signals.
RF signal pre-distortion is another approach. The objective in this approach is to directly cancel the distortion of the power amplifier by pre-distorting the signal going into it. As in the case of the baseband pre-distortion scheme, the RF signal pre-distortion can be adaptive using a cancellation scheme based on the transmitter signal.
Namely, some transmitters use a cancellation scheme as a variation of the pre-distortion approach. This involves adaptive feedback where the gain of cancellation amplifiers is adaptively modified. Adaptive control methods are used to adjust a distortion canceling circuitry for changing conditions such as transmitter power levels, temperature, or aging. Examples of methods for adaptive control include conversion to baseband, conversion to IF, and predictive calibration.
In the conversion to baseband scheme, a simplified receiver (located in the transmitter) is used to sample the modulated transmit signal. The distortion canceling circuit is adjusted based on the demodulated baseband signal. The gain adjustment combined with the pre-distortion reduces amplitude and phase distortions. However, this approach assumes that the distortion is a relatively small component of the signal. Moreover, frequency changes would negate the corrective effects of the cancellation scheme.
In the conversion to IF scheme, the modulated output signal (usually from the transmitter power amplifier) is down converted to an IF signal. The modulated IF signal is filtered to monitor the amount of distortion. Then a distortion canceling circuit is adjusted to minimize the distortion. This approach suffers from deficiencies similar to those outlined above.
With predictive calibration, the transmitter uses a look-up table based on temperature and transmitter power level to adjust the distortion canceling circuit. This is not a true adaptive method, but an open loop technique requiring careful characterization of the transmitter.
Over time, numerous combinations of distortion canceling and adaptive control methods have been broached. Commonly, these techniques have been employed in the transmitter sections of wireless digital communications systems.
However, given that distortion remains a factor in digital transmission, design considerations of dynamic range, bit error rate, power efficiency, data rate capacity and the like also remain. Accordingly, in dealing with the associated design challenges a better approach is needed.